Array.prototype.myMap = function(fn,array ) {
  let res = []
  let arr = this
  console.log(this)
  for(let i in arr) {
    console.log(i)
     res.push(fn(arr[i]))
  }
  return res
}
const a = [1, 2, 3, 4];
const b = a.myMap(x => x * 2);
console.log(b); 
//this有问题